1. Field of the Invention
This invention relates to packaging of microwave circuits in the high microwave frequencies and in applications using hermetically sealed packages for monolithic microwave integrated circuits, and more particularly to such packages having vias for transmitting signals into and out of the packages.
2. Description of Related Art
Ultra high speed monolithic microwave integrated circuits (MMIC), microwave integrated circuits (MIC), other integrated circuits and hybrid circuit dies are mounted in environmentally protective or hermetically sealed packages that provide electromagnetic shielding and easy handling. Known manufacturing techniques include cofired ceramic enclosures using thick or thin film metallization, glass or quartz seals, ceramic enclosures using thin-film metallization, metal enclosures having ceramic feedthroughs, and metal enclosures having glass feedthroughs.
In ceramic packages generally available for MMIC's, the main contributor to poor microwave performance is the feedthrough. The insertion loss of a coaxial line or stripline formed on the feedthrough through a hermetically sealed ceramic wall increases with higher frequency, which results in a diminished signal strength. High insertion loss degrades MIC performance in many ways such as increased noise figure of small signal devices and reduced output power and efficiency of a power amplifier. An MMIC package capable of good performance in the microwave range should have low insertion loss per feedthrough.
U.S. Pat. No. 5,117,068 describes a conventional packaging assembly having vias in the substrate for transmitting signals into and out of the package. As shown in FIGS. 1A-1C (exploded, perspective and cross-sectional views, respectively), the package 10 comprises a lead frame 11, leads 12 an alumina base or substrate 13 having solid metal vias 14 extending therethrough, a seal-ring wall 15 and a cover 16 mounted together. The top surface of the substrate 13 is plated with a conductive film 17.
FIGS. 2A-2C show a conventional via structure for the type of packages shown in FIGS. 1A-1C. FIGS. 2A and 2C are the top and bottom views of the via structure, and FIG. 2B is a cross-sectional view along the line 2B--2B in FIG. 2A. A first metallization layer 201 is formed on the top or inside surface of the substrate 210 to serve as the ground of the device mounted in the package. A wire bond pad metallization 202 is formed over and connected to the lead via 203 and is separated from the rest of the metallization layer 201 by a gap 204. A second metallization layer 205 is formed on the bottom or outside surface of the substrate 210 as the ground. The ground metallization layers 201 and 205 on the top and bottom surfaces of the substrate 210 are electrically connected by a plurality of ground vias 206 that pass through the substrate. A lead pad metallization 207 is formed on the bottom surface separated from the ground metallization layer 205 by a gap 209 and is connected to the lead via 203. A ground plane 212 is connected to the ground metallization layer 205, and a lead 208 is connected to the lead pad 207 and extends outwardly under the seal-ring wall 211 over the edge of the substrate 210.